Wafer area pressure control for plasma confinement

ABSTRACT

A plasma processing chamber is provided which provides improved wafer area pressure control. The plasma processing chamber is a vacuum chamber with a device connected for generating and sustaining a plasma. Part of this device would be an etchant gas source and an exhaust port. A confinement ring defines an area above a wafer. The wafer area pressure is dependent on the pressure drop across the confinement ring. The confinement ring is part of a wafer area pressure control device that provides wafer area pressure control range greater than 100%. Such a wafer area pressure control device may be three adjustable confinement rings and a confinement block on a holder that may be used to provide the desired wafer area pressure control.

RELATED APPLICATIONS

This application is related to the commonly assigned U.S. patentapplication Ser. No. 09/637,736 entitled WAFER AREA PRESSURE CONTROL, byFangli Hao, Eric Lenz, and Bruno Morel.

BACKGROUND OF THE INVENTION

The present invention relates to the processing equipment for thefabrication of semiconductor-based devices. More particularly, thepresent invention relates to improved techniques for confining andcontrolling the pressure of the plasma in plasma processing chambers.

In the fabrication of semiconductor-based devices (e.g., integratedcircuits or flat panel displays) layers of material may alternately bedeposited onto and etched from a substrate surface (e.g., thesemiconductor wafer or glass panel). As is well known in the art, theetching of the deposited layer(s) may be accomplished by a variety oftechniques including plasma-enhanced etching. In plasma-enhancedetching, the etching of the deposited layer(s) on the substrate takesplace inside a plasma processing chamber. During etching, a plasma isformed from a suitable etchant gas source to etch areas of the depositedlayer(s) on the substrate that are unprotected by the mask, leavingbehind the desired pattern.

Among different types of plasma etching systems, those utilizing methodsto confine the plasma to a volume immediately above the substrate haveproven highly suitable for efficient production and/or for forming theever-shrinking features on the substrate. An example of such a systemmay be found in commonly assigned U.S. Pat. No. 5,534,751, which isincorporated by reference herein. Although plasma confinement results ina significant improvement in the performance of plasma processingsystems, current implementations can be improved. In particular, it isrealized that improvements can be made in the control of the pressure ofthe confined plasma and the accessibility of the plasma processingvolume for substrate transport.

To facilitate discussion, FIG. 1A depicts an exemplary plasma processingchamber 100, including confinement rings 102 as they are currentlyimplemented. Within plasma processing chamber 100, the substrate 106 ispositioned upon the lower electrode 104. The lower electrode 104incorporates a suitable substrate chucking mechanism (e.g.,electrostatic, mechanical clamping, or the like) for holding thesubstrate 106. The reactor top 110 incorporates an upper electrode 112disposed immediately opposite the lower electrode 104. The upperelectrode 112, lower electrode 104, and confinement rings 102 define theconfined plasma volume 116. Gas is supplied to the confined plasmavolume 116 by etchant gas source 114 and is exhausted from the confinedplasma volume 116 through the confinement rings 102 and exhaust port 120by a vacuum pump. With gas flowing and an appropriate pressureestablished within the confined plasma volume, a plasma is formed withinthis volume by application of RF power to the lower electrode by RFsource 108 while grounding upper electrode 112. Alternately, as is wellknown in the art, the plasma may be formed by applying RF power to bothlower electrode 104 and upper electrode 112, or by grounding lowerelectrode 104 and applying RF power to upper electrode 112.

The confinement rings 102 serve both to confine the plasma to the volume106 and to control the pressure of the plasma. The confinement of theplasma to the volume 116 is a function of many factors including thespacing between the confinement rings 102, the pressure in the volumeoutside the confinement rings and in the plasma, the type and flow rateof the gas, as well as the level and frequency of RF power. Foreffective plasma confinement, the pressure outside the confinement rings102 should be as low as possible, preferably less than 30 millitorr.Confinement of the plasma is more easily accomplished if the spacingbetween the confinement rings 102 is very small. Typically, a spacing of0.15 inches or less is required for confinement. However, the spacing ofthe confinement rings also determines the pressure of the plasma, and itis desirable that the spacing can be adjusted to achieve the pressurerequired for optimal process performance while maintaining plasma.

Commonly assigned U.S. Pat. No. 6,019,060 entitled “Cam-BasedArrangement for Positioning Confinement Rings In A Plasma ProcessingChamber” by Eric H. Lenz, issued Feb. 1, 2000, incorporated byreference, taught that the pressure drop across the confinement rings isapproximately proportional to the expression 1/(X²+Y²+Z²) where X, Y andZ are the distances between confinement rings as shown in FIG. 1B. Lenzprovided a single movable ring and a stationary ring (X=constant,Y+Z=constant in FIG. 1B). By adjusting the distances Y and Z by movingthe single movable confinement ring, as taught by Lenz, a plasmapressure control range can be obtained. FIG. 2 illustrating the relativepressure predicted by the expression above obtained by moving a singlering for of various fixed gaps, X. The expression predicts a. controlrange of 67-100% can be obtained, as illustrated in FIG. 2, whileexperiments found the achievable range to be approximately one halfthose values. In many cases, a wider plasma pressure range is requiredto achieve optimal process results on various types of films and deviceswithin the same processing system.

In addition, in the method taught by Lenz, the confinement rings 102 areconstrained between the upper and lower electrode assemblies and thuscan restrict access to the interelectrode space for loading andunloading of substrates. As shown in FIG. 1C, even with the confinementrings 102 lifted to their uppermost position, the access to theinterelectrode space is limited to the gap W, which is the difference ofthe total interelectrode space less the combined thicknesses of theconfinement rings.

It is desirable to provide an increased range of pressure control whilemaintaining plasma confinement. It is also desirable to provideconfinement rings that greater facilitate placement and removal of thesubstrate from the plasma processing system.

SUMMARY OF THE INVENTION

To achieve the foregoing and other objects and in accordance with thepurpose of the present invention, a plasma processing device isprovided. A vacuum chamber with an exhaust port and vacuum pump in fluidconnection with the vacuum chamber and a gas source in fluid connectionwith the vacuum chamber is provided. Within the vacuum chamber a waferarea pressure control device for providing wafer area pressure controlrange greater than 500% is placed.

In addition, the present invention provides a method of controllingwafer area pressure. Generally, a substrate is placed in a vacuumchamber. A gas source is provided to the vacuum chamber. Gas is alsoexhausted from the vacuum chamber. At least one ring is moved to providewafer area pressure control range greater than 500%.

These and other features of the present invention will be described inmore detail below in the detailed description of the invention and inconjunction with the following figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a schematic view of a prior art plasma processing chamber.

FIG. 2 is a graph illustrating the relative pressure achieved with theprior art.

FIG. 3 is a schematic view of a plasma processing chamber according toone embodiment of the invention.

FIG. 4 is a flow chart of the operation of the preferred embodiment ofthe invention.

FIG. 5 is a schematic view of a section of the plasma processing chambershown in FIG. 3 where the confinement rings are in their uppermostposition.

FIG. 6 is schematic view of the plasma processing chamber shown in FIG.5, where the confinement rings have been lowered.

FIG. 7 is schematic view of the plasma processing chamber shown in FIG.6, where the confinement rings have been lowered further so as to reducethe lowermost gap to its minimum.

FIG. 8 is schematic view of the plasma processing chamber shown in FIG.7, where the confinement rings have been lowered farther so as to reducethe middle gap to its minimum.

FIG. 9 is schematic view of the plasma processing chamber shown in FIG.8, where the confinement rings have been lowered further so as to reducethe uppermost gap to its minimum.

FIG. 10 is a graph of pressure with respect to the sum of the gap sizes.

FIG. 11 is a graph of pressure with respect to the sum of the gap sizesfor various values of minimum gap size.

FIG. 12 is a schematic view of a plasma processing chamber of anotherembodiment of the invention.

FIG. 13 is a schematic view of part of another embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference toa few preferred embodiments thereof as illustrated in the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one skilled in the art, thatthe present invention may be practiced without some or all of thesespecific details. In other instances, well known process steps and/orstructures have not been described in detail in order to notunnecessarily obscure the present invention.

To facilitate discussion, FIG. 3 is a cross-sectional view of a plasmaprocessing chamber 200. The chamber has a top 212 and a bottom 214, anda chamber wall 216 extending from the top 212 to the bottom 214.Cantilevered from one side of the chamber is a lower electrode housing218 in which is fixtured a chuck 204, which is a workpiece holder onwhich a substrate 206 is positioned during etching. The chuck 204 may beimplemented by any suitable chucking technique, e.g., electrostatic,mechanical clamping, vacuum, or the like. An RF power supply 252 may beelectrically connected to the chuck 204. The reactor top 212 supports anupper electrode 224 and may be electrically connected to an RF powersupply. A confinement mechanism comprising a first adjustableconfinement ring 230, a second adjustable confinement ring 232, a thirdadjustable confinement ring 234, and a confinement block 236 aredisposed within the processing chamber 200. A holder 240 may be used tosupport the first adjustable confinement ring 230, second adjustableconfinement ring 232, third adjustable confinement ring 234, andconfinement block 236. A controller 242 connected to the holder 240controls the movement of the holder 240 and therefore the firstadjustable confinement ring 230, second adjustable confinement ring 232,third adjustable confinement ring 234, and confinement block 236. In thepreferred embodiment of the invention, the holder 240 is stepped with aconfinement ring or block resting on each step. The steps provide amaximum gap spacing between the confinement rings and block, which inthe preferred embodiment of the invention is between 0.09 to 0.15 inches(2.28 to 3.81 mm). The first adjustable confinement ring 230, the secondadjustable confinement ring 232 and the third adjustable confinementring 234 each are fixtured with spacers 238, which fix the minimum gapbetween each confinement ring and block. In this embodiment the spaceris of a size to provide a minimum gap between 0.005 and 0.060 inches(0.13 to 1.52 mm). An etchant gas source 250 provides gas to thechamber. A pressure sensor 262 measures the pressure in the volume abovethe substrate 206, the “Wafer Area Pressure”, or “WAP”. The chamber 200has an exhaust port 260.

In operation of this embodiment, the controller 242 raises the holder240 to its highest position as shown in FIG. 3 and in detail in FIG. 5.Such a position lifts the first adjustable confinement ring 230, secondadjustable confinement ring 232, third adjustable confinement ring 234,and confinement block 236 to a height such that the gap between theunderside of the first containment ring 230 and the plane of the chuck204 is, at a minimum, of size sufficient to allow the robotic placementof the substrate 206 onto the chuck 206. In the preferred embodiment,this gap is on the order of 0.5 inches (12 mm).

FIG. 4 is a flow chart of the operation of the preferred embodiment ofthe invention. In operation, the holder raises the first adjustableconfinement ring 230, second adjustable confinement ring 232, thirdadjustable confinement ring 234, and confinement block 236 to theposition shown in FIG. 5 (step 302). A robotic mechanism may be used toplace the substrate 206 on the chuck 204 (step 304). The controller 242lowers the holder 240 to a wafer area pressure control starting point(step 306). The wafer area pressure control starting point may be theposition of the first adjustable confinement ring 230, second adjustableconfinement ring 232, third adjustable confinement ring 234, andconfinement block 236 that allows minimal pressure drop whilemaintaining sufficient confinement during processing. In this embodimentthe wafer area pressure control starting point may be when the firstadjustable confinement ring 230 begins resting on the lower electrodehousing 218, as shown in FIG. 6. Etchant gas flow is then initiated(step 308) and the pressure within the wafer area as measured by thepressure sensor 252 is compared to a desired setpoint value (steps 310).If the wafer area pressure needs to be increased (step 312), thecontroller 242 may further lower the holder 240 (step 316). FIG. 7illustrates a chamber 200 where the holder 240 is lowered to a pointwhere the second confinement ring 232 rests on the spacers 238 of thefirst confinement ring 230. FIG. 8 illustrates a chamber 200 where theholder 240 is lowered further to a point where the third confinementring 234 rests on the spacers 238 of the second confinement ring 232.FIG. 9 illustrates a chamber 200 where the holder 240 is lowered furtherstill to a point where the confinement block 236 rests on the spacers238 of the third confinement ring 234. This is the lowest wafer areacontrol position, and the holder 240 is not lowered any further.

If the wafer area pressure needs to be decreased, the controller 242 mayraise the holder 240 (step 314). Once the holder 240 reaches the highestwafer area pressure control position, as shown in FIG. 6, the holder 240is not raised any further during the wafer area pressure control stage.Once the wafer area pressure is equal to the desired setpoint value, theplasma is initiated (step 318). The wafer area pressure is againcompared against a setpoint (step 320) and the position of theconfinement rings adjusted by raising or lowering the holder 240 (steps322, 324, 326) to achieve and maintain the desired pressure until theplasma process is deemed complete (step 330), whereupon the holder 240is raised to its highest position (step 332), and the robitic mechanismis used to remove the substrate (step 334), so that the process may berepeated.

In the preferred embodiment the change in the pressure drop across theconfinement rings as the holder 240 is lowered from its highest controlposition, as shown in FIG. 6, to its lowest control position, as shownin FIG. 9, may change by 300-800%. Control of the pressure drop isprovided in that to increase wafer area pressure the holder is moveddownward and to decrease pressure the holder is moved upward. As shownin FIG. 9, the confinement block 236 is of sufficient thickness suchthat that the top of the confinement block 236 is above the lowest partof the upper electrode 224 so as to prevent exiting the wafer area byflowing over the top of the confinement block 236.

In an example of the dimensions that may be used in this embodiment, thegap between the confinement block 236 and the upper electrode 224, whichis surrounded by the confinement block 236, may be between 0.0125 and0.0500 inches (0.32 to 1.27 mm). More preferably, the gap is about 0.025inches (0.63 mm). The thickness of the first, second, and thirdadjustable confinement rings 230, 232, 234 are between 0.045 to 0.180inches (1.14 to 4.57 mm). More preferably the thickness is about 0.09inches (2.29 mm). The distance between the upper electrode 224 and thelower electrode 204 may be in the range of 0.4 to 3.0 inches (10 mm to76.2 mm). More preferable the distance is about 0.6 inches (15 mm).

FIG. 10 is a graph of wafer area pressure with respect to the sum of thegap distances in inches between the first adjustable confinement ring230 and second adjustable confinement ring 232; the second adjustableconfinement ring 232 and third adjustable confinement ring 234; and thethird adjustable confinement ring 234 and confinement block 236. In thisexample, the pressure is measured with a flow of 300 standard cubiccentimeters per minute (sccm) of Argon being fed into the chamber 200.The square data points 902 represent wafer area pressure, which ispressure within the confinement rings. The diamond data points 904represent the pressure of the chamber 200. Section (I) corresponds tothe movement of the confinement rings from the position shown in FIG. 6to the position shown in FIG. 7. Section (II) corresponds to movement ofthe confinement rings from the position shown in FIG. 7 to the positionshown in FIG. 8. Section (III) corresponds to the movement of theconfinement rings from the position shown in FIG. 8 to the positionshown in FIG. 9.

FIG. 11 is a graph of the wafer area pressure with respect to the sum ofthe gap distances in inches corresponding to Sections (II) and (III) inFIG. 10, but for differing values of minimum gap distances as set by thespacers 236. In Curve (a), the minimum gap distance as set by thespacers 236 is 0.007 inches for the gap between the first (230) andsecond (232) adjustable confinement rings and 0.007 inches for the gapbetween the second (232) and third (234) adjustable confinement rings.In Curve (b) the minimum gap distance as set by the spacers 236 is 0.030inches for the gap between the first 230 and second 232 adjustableconfinement rings and 0.007 inches for the gap between the second 232and third 234 adjustable confinement rings. In Curve (c) the minimum gapdistance as set by the spacers 236 is 0.038 inches for the gap betweenthe first 230 and second 232 adjustable confinement rings and 0.030inches for the gap between the second 232 and third 234 adjustableconfinement rings. In Curve (d) the minimum gap distance as set by thespacers 236 is 0.038 inches for the gap between the first 230 and second232 adjustable confinement rings and 0.038 inches for the gap betweenthe second 232 and third 234 adjustable confinement rings. In Curve (e)the minimum gap distance as set by the spacers 236 is 0.062 inches forthe gap between the first 230 and second 232 adjustable confinementrings and 0.062 inches for the gap between the second 232 and third 234adjustable confinement rings. From the graph in FIG. 11 it can be seenthat the pressure control range and the slope of the pressure controlcan be changed by the use of different spacers 236 to change the minimumgap distance.

The holder may be any device that allows one or more controllers toprovide an upward and downward movement of the confinement rings andconfinement blocks, where in a raised position the holder maintains amaximum gap between the confinement rings and the confinement block andin a lowered position the holder allows the confinement rings andconfinement block to form minimum gaps. Preferably, the holder allowsone gap at a time to decrease until the gap is minimized. Once a gap isminimized, another gap is decreased. The holder 240 in FIG. 5 is a typeof hanger, since the holder 240 is below the controller 242. In anotherembodiment, the holder 240 could be configured so as to be above thecontroller 242, and the holder 240 is now a type of platform.

Although three confinement rings and a confinement block are shown inthe preferred embodiment, other numbers of confinement rings and blocksmay be used. In the preferred embodiment, the number and spacing of theconfinement rings and confinement block are such that at the highestposition, at least part of the lowest confinement ring is above thelowest part of the upper electrode and, for a wafer area pressurecontrol initial position, the lowest confinement ring rests on a surfacecoplanar with the lower electrode and the confinement rings and aportion of the confinement block are below the upper electrode. Thisembodiment helps to avoid stagnation points, which may cause theaccumulation of deposited polymer films.

In another embodiment, the holder may comprise a plurality of hangerswith each hanger suspending a different confinement ring. FIG. 12illustrates parts of a first adjustable confinement ring 1204, a secondadjustable confinement ring 1208, a third adjustable confinement ring1212, and a confinement block 1216. A first hanger 1220 is suspendedfrom a controller 1224. The confinement block 1216 hangs from the firsthanger 1224. A second hanger 1228, a third hanger 1232, and a fourthhanger 1236 hang from the confinement block 1216. The second hanger 1228supports the third adjustable confinement ring 1212. The third hanger1232 supports the second adjustable confinement ring 1208. The fourthhanger 1236 supports the first adjustable confinement ring 1204. Thesecond, third, and fourth hangers allow the first, second, and thirdadjustable confinement rings to stop moving when they reach their lowestpoint, as described in the previous embodiment.

In yet another embodiment, the profile of the confinement ring may becomplex. FIG. 13 illustrates parts of a first adjustable confinementring 1330, a second adjustable confinement ring 1332, a third adjustableconfinement ring 1334, and a confinement block 1336. A holder 1340 maybe used to support the first adjustable confinement ring 1330, secondadjustable confinement ring 1332, third adjustable confinement ring1334, and confinement block 1336. A controller 1342 connected to theholder 1340 controls the movement of the holder 1340 and therefore thefirst adjustable confinement ring 1330, second adjustable confinementring 1332, third adjustable confinement ring 1334, and confinement block1336. The holder 1340 is stepped with a confinement ring or blockresting on each step. The steps define the maximum gap spacing betweenthe confinement rings and block. The first adjustable confinement ring1330, second adjustable confinement ring 1332, and third adjustableconfinement ring are fixtured with spacers 1338, which define theminimum gap spacing between adjacent confinement rings. The matingsurfaces of the confinement rings 1330, 1332, 1334 and the confinementblock 1336 are not flat, but instead incorporate a “profile” such that,at the maximum gap spacing, the gaps are “optically dense”, that is theydo not provide a direct line of sight, which may serve to improveconfinement of the plasma by minimizing the streaming of chargedparticles from the plasma, through the gaps into the volume outside theconfinement rings. FIG. 13 illustrates one embodiment of this “profile”,consisting of a vertical offset 1344 in each mating surface of amagnitude greater than one half the maximum gap spacing. The offset ineach mating surface is radially staggered to allow each mating surfaceto be brought into close proximity with the next.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and substituteequivalents, which fall within the scope of this invention. It shouldalso be noted that there are many alternative ways of implementing themethods and apparatuses of the present invention. It is thereforeintended that the following appended claims be interpreted as includingall such alterations, permutations, and substitute equivalents as fallwithin the true spirit and scope of the present invention.

What is claimed is:
 1. An apparatus for controlling pressure,comprising: a vacuum chamber; an exhaust port in fluid connection withthe vacuum chamber; a gas source in fluid connection with the vacuumchamber; and a wafer area pressure control device, wherein the waferarea pressure control device comprises: a first adjustable confinementring within the vacuum chamber; a second adjustable confinement ringwithin the vacuum chamber; an adjustable confinement block within thevacuum chamber; and a controller for raising and lowering the firstconfinement ring, the second confinement ring, and the confinementblock.
 2. The apparatus, as recited in claim 1, wherein the wafer areapressure control device, further comprises at least one holder connectedbetween the controller and the first adjustable confinement ring, thesecond adjustable confinement ring, and the adjustable confinementblock.
 3. The apparatus, as recited in claim 2, wherein the at least oneholder moves the first adjustable confinement ring, the secondadjustable confinement ring, and the adjustable confinement block. 4.The apparatus for controlling pressure, comprising: a vacuum chamber; anexhaust port in fluid connection with the vacuum chamber; a gas sourcein fluid connection with the vacuum chamber; and a wafer area pressurecontrol device, wherein the wafer area pressure control devicecomprises: a first adjustable confinement ring within the vacuumchamber; a second adjustable confinement ring within the vacuum chamber;an adjustable confinement block within the vacuum chamber; a controllerfor raising and lowering the first confinement ring, the secondconfinement ring, and the confinement block; and at least one holderconnected between the controller and the first adjustable confinementring, the second adjustable confinement ring, and the adjustableconfinement block, wherein the at least one holder moves the firstadjustable confinement ring, the second adjustable confinement ring, andthe adjustable confinement block; an upper electrode, and wherein the atleast one holder is able to move at least part of the first adjustableconfinement ring, the second adjustable confinement ring and theadjustable confinement block to a position above a lower surface of theupper electrode.
 5. The apparatus, as recited in claim 4, wherein the atleast one holder is able to move the first adjustable confinement ringto rest on a surface on a plane with or below that of the substrate. 6.The apparatus, as recited in claim 5, further comprising a spacerbetween the first adjustable confinement ring and the second adjustableconfinement ring.
 7. The apparatus, as recited in claim 6, wherein theat least one holder is able to move the second adjustable confinementring from a position spaced apart from the first confinement ring toform a maximum spacing to a position wherein the second adjustableconfinement ring rests on the first adjustable confinement ring,separated by the spacer.
 8. The apparatus, as recited in claim 7,wherein the wafer area pressure control device further comprises a thirdadjustable confinement ring located between the second adjustableconfinement ring and the adjustable confinement block and connected tothe holder.
 9. An apparatus for controlling pressure, comprising: avacuum chamber; an exhaust port in fluid connection with the vacuumchamber; a gas source in fluid connection with the vacuum chamber; and awafer area pressure control device, wherein the wafer area pressurecontrol device comprises: a first adjustable confinement ring; a secondadjustable confinement ring; a third adjustable confinement ring; acontroller for raising and lowering the first adjustable confinementring, the second adjustable confinement ring, and the third adjustableconfinement ring; and at least one holder connected to the controller,the first adjustable confinement ring, the second adjustable confinementring, and the third adjustable confinement ring.
 10. The apparatus, asrecited in claim 9, wherein when the at least one holder is lowered to alowest position the wafer area control device provides a highestpressure drop and wherein when the at least one holder is raised to ahighest position the wafer area control device provides a lowestpressure drop.
 11. The apparatus, as recited in claim 10, wherein thepressure drop of the wafer area control device continuously increaseswhen the at least one holder is moved from the highest position to thelowest position.
 12. The apparatus as recited in claim 11, wherein theat least one holder comprises: a first holder connected to thecontroller and the first adjustable confinement ring; and a secondholder connected to the controller and the second adjustable confinementring.
 13. The apparatus, as recited in claim 1, wherein the wafer areapressure control device, further comprises at least one holder connectedto the controller and the first adjustable confinement ring, the secondadjustable confinement ring, and the adjustable confinement block. 14.The apparatus, as recited in claim 13, wherein when the at least oneholder is lowered to a lowest position the wafer area control deviceprovides a highest pressure drop and wherein when the at least oneholder is raised to a highest position the wafer area control deviceprovides a lowest pressure drop.
 15. The apparatus, as recited in claim14, wherein the pressure drop of the wafer area control devicecontinuously increases when the at least one holder is moved from thehighest position to the lowest position.
 16. The apparatus as recited inclaim 15, wherein the at least one holder comprises: a first holderconnected to the controller and the first adjustable confinement ring;and a second holder connected to the controller and the secondadjustable confinement ring.
 17. The apparatus as recited in claim 14,wherein the at least one holder comprises: a first holder connected tothe adjustable confinement block and the first adjustable confinementring; a second holder connected to the adjustable confinement block andthe second adjustable confinement ring; and a third holder connected tothe controller and the adjustable confinement block.